MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. 399 
have been so ready to break forth at a sudden accession of water from below; and the 
rainfall not having been great, there might not have existed such a plentiful source of 
underground water to be drawn from. The following review of the rainfall conditions 
may be of value in indicating a relation between the preparedness of the ground and 
the number and importance of flows and slumps. 
During the first three months of 1906 the rainfall was exceptionally heavy thruout 
California, being on an average thruout the whole State more than 9 inches in excess 
of the normal for that period. Up to the beginning of 1906, the amount of rain for 
the season was 4.5 inches below the average; but owing to the great excess during the 
late winter and early spring months the total for the year up to the first of April, the 
month in which the earthquake occurred, was nearly 5 inches above the normal. During 
January, February, and. March the rain was heavy and continuous. Nearly all the 
rain of the season was during these months immediately preceding the earthquake 
month. Practically no rain fell between April 1 and April 18. 
All of the rainfall data available in the monthly reports of the Weather Bureau for 
California, compiled by Professor McAdie, has been used for calculating the amount of 
rain in 8 counties south of San Francisco. These are San Francisco, Alameda, San 
Mateo, Santa Clara, Santa Cruz, Monterey, San Luis Obispo, and Santa Barbara. The 
average rainfall at 46 different places distributed thru these counties was 22.59 inches 
from September, 1905, to April 1, 1906, between 2 and 3 inches above the normal for 
this region. The excess would have been greater but for the lightness of the rainfall 
during the autumn term, which was 3.55 inches, or several inches less than the average 
for former years. During the spring season up to April 1, the precipitation was exces- 
sive. During the three months that preceded the earthquake, 19.04 inches of rain fell, 
or 84.30 per cent of the whole precipitation up to that time. During the first half of 
April, there was practically no rain at all. Thruout this region, as well as thru Cali- 
fornia as a whole, March was a very rainy month; especially heavy downpours coming 
everywhere in the State during the last days of the month. It was the rainiest of the 
months except in parts of Santa Clara and Santa Cruz Counties, where more fell in the 
month of January. 
The majority of the earth-flows and earth-slumps that occurred were near the coast, 
although the amount of rain that fell was not as large there as it was farther back in the 
mountains. The coast region, however, is subject to heavy fogs, which precipitate some 
moisture and help to prevent evaporation of the moisture already present. These fogs 
were probably a factor in causing the earth-flows and earth-slumps near the sea. The 
principal cases described were near Half Moon Bay. The records from Point Montara, 
only a few miles away, showed that the rainfall in this vicinity was heavier than at any 
other point along the coast south of San Francisco. During the spring season up to 
April 1, it amounted to 23 inches, and during the autumn season it amounted to 12 
inches. The table shows that the heaviest rains were in the Santa Cruz Mountains. 
At Boulder Creek, in Santa Cruz County, 55.70 inches of rain fell during January, 
February, and March alone, and 16 inches fell during the four months preceding. 
During the spring of 1906, a large part of the precipitated moisture remained in the 
ground, which was previously dry, and the amount of evaporation was minimized by 
the continuous succession of cloudy and rainy days. The year afforded an example of 
the concentration of an excessive annual rainfall into a short period, with all the con- 
ditions favorable for the absorption and retention of the moisture in the ground. For 
this reason, conditions favored the production of debacles of various kinds in the loose 
material covering slopes. 
The earth-flows that have been discust are more or less similar to the flows occasioned 
by the bursting of peat-bogs. The causes of their origin and their nature appear to be 
